In the field of color photographic paper, demands for performances such as higher sensitization, stabilization of processing, the improvement of image quality and the improvement of development processing speed have been remarkably increased in recent years. In higher sensitization of a silver chloride emulsion, the incorporation of a bromide ion into the emulsion grain has been primarily discussed. For example, a method of incorporating a bromide ion into the surface of a grain is disclosed in EP 0295439, a method of incorporating an epitaxially formed local phase having a bromide concentration of at least 20 mol % into the grain surface is disclosed in U.S. Pat. No. 5,252,454, and a method of blending a fine grain emulsion with a high silver chloride host grain emulsion having larger grain size and performing Oatwald ripening to thereby form a bromide rich phase on the surface of a grain is disclosed in U.S. Pat. No. 5,252,456. On the other hand, sensitivity is also increased by incorporating an iodide ion into a silver chloride emulsion grain. However, the examples of using iodide ions for the purpose of higher sensitization are not so many hitherto and, as represented by the above-cited patents, iodide ion is in general permitted only in low concentration or avoided.
Some examples of achieving higher sensitization by the incorporation of an iodide ion in a high silver chloride emulsion are shown below. Techniques of forming a high silver chloride (100) tabular grain and then incorporating an iodide ion into the grain in the state of a band are disclosed in U.S. Pat. Nos. 5,264,337, 5,292,632 and 5,314,798. An example of epitaxially adhering a bromide ion and a low iodide contention in combination on the corners of a tabular grain is disclosed in U.S. Pat. No. 5,275,930. JP-A-8-220681 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-8-234340, JP-A-8-220684, JP-A-8-240879 and JP-A-8-234345 disclose in common various designs of the incorporation of an iodide ion into a high silver chloride host grain and these patents contrive to have the largest iodide ion concentration in the sub-surface of an emulsion grain and form intentionally a high silver chloride layer not containing an iodide on the layer containing an iodide.
When an iodide ion is subjected to mixed-crystallization to a silver chloride, silver ions between lattices of the inside of a grain conspicuously increase. Therefore, the improvement of sensitization by an iodide ion seems as the acceleration of formation of a light-sensitive speck due to the increment of silver ions between lattices. Since there is not the layer containing an iodide on the surface or an iodide is limited to the corners in the above-exemplified patents, the maximum of the effect of higher sensitization cannot be brought out. Contrary to this, when an iodide ion is contained only on the outermost surface of a grain, the iodide ion adsorbed onto the outermost surface of the grain after grain formation is unstably incorporated into a silver chloride layer by the process after that, e.g., by chemical sensitization, as a result, fog is liable to occur hence unsuitable for the achievement of higher sensitization.